sendToBack method
Moves an object to the back of z order.
Implementation
bool sendToBack(String id) {
final index = _objects.indexWhere((object) => object.id == id);
if (index <= 0) return false;
_remember();
final object = _objects.removeAt(index);
_objects.insert(0, object);
render();
_emitChange();
return true;
}